CLEANING METHOD

Abstract: This invention relates to a method of cleaning electronic components using a water-miscible solvent system. In the method, the components are washed with a water-miscible solvent system, rinsed with water and the used solvent and water entraining the impurities are combined and passed through an absorptive medium which retains all the suspended impurities but does not absorb the solvent or water. The cleaned aqueous solvent can be recovered and concentrated by evaporating off the water to leave behing a concentrated solvent solution. Use of this method minimises emission of undesirable inorganic, organic and/or particulate materials into the environment.

Description: CLEANING METHOD This invention relates to a method of cleaning electronic components using a solvent system in a manner to minimise emission of (in)organic compounds and particulate materials into the environment. It is well known to use various solvents and solvent blends as a cleaning agent for the removal of oil and other contaminants from substrates such as drill cuttings etc. Most of these solvents cannot be used for the removal of inorganic/organic fluxes or other contaminants encountered on electronic circuit boards and printed circuit boards. Removal of flux from such circuit boards is not only necessary for cosmetic reasons but also to prevent corrosion by such fluxes on the boards and to allow automatic testing of such boards. Whilst conventional solvents used for this purpose have included chlorofluorocarbons, the so-called "CFC's", our published EP-A- 0523892 claims and describes a process for using oxygenated solvents for this purpose thereby rendering such solvents environmentally more acceptable. In use, however, such solvents as are claimed eg in our published EP-A-0523892, can be rendered more efficient and more environmentally friendly if these solvent could be recovered and re- used. The difficulty with this has been that such solvents are invariably rinsed off the surfacer -if the cleaned components with water and the water discharged. Aiso, due to the efficiency of the solvent, there is a rapid build up of contamination in rinse tanks, especially due to the carry over of the contaminants from one tank to the next. If the cleaning solvent is changed out when the final rise reaches an unacceptable level (typically 1000 ppm flux) then the process would be uneconomic. An extension of all such cleaning baths can be achieved in two ways: (a) by increasing the number of tanks (effectively increasing the degree of dilution of the contamination thereby adding significantly to the size and cost of the equipment, or, (b) by removal of contamination from the final rinse thereby increasing the running costs. The latter method, whilst preferred by the electronics industry is not so simple. For instance, the solvents is usually mixed with substantial amounts of rinse water and the solvent/rinse water mixture is difficult to separate not only from each other but also from other suspended impurities since use of absorbents to achieve this have invariably results in said absorbent removing the solvent at the same time as the other suspended impurities thereby resulting in loss of valuable solvents. Moreover, some of the conventional absorbents such as the commercial water purification carbons have little capacity for flux residues. It is an object of the present invention to provide a method for the cleaning of such electronic components using a water-miscible solvent system and wherein the solvent is separated from the water and the suspended impurities whether, particulate or colloidal, so as to be recovered and re-used. Accordingly, the present invention is a method of cleaning electronic components comprising the steps of: a. cleaning said component(s) in a water-miscible organic solvent followed by rinsing said components so cleaned at least once with water to generate an aqueous effluents comprising the solvent, water and suspended impurities from the cleaning and rinsing stages; and b. passing the aqueous effluent through an absorptive medium capable of removing the suspended impurities but not the aqueous solvent. By the expression "electronic components" as used herein and throughout the specification is meant to include inter alia electronic circuit boards, printed circuit boards, precision engineered components and the like which have been exposed to solders, fluxes and/or oil treatments during the production thereof. By the expression "aqueous effluent" is meant here and throughout the specification the effluent comprising the water- miscible cleansing solvent and water whether generated during cleaning or rinsing cycles of the method which may or may not include suspended impurities, whether they be particulate, colloids or emulsions, and dissolved impurities including soluble cationic and anionic impurities. By the expression "suspended impurities" is meant here and throughout the specification, impurities which may be in particulate, colloidal or emulsion form but does not include dissolved impurities. The electronic components may be cleaned either by immersion of the soiled components in the water-miscible solvent for a suitable duration and/or by spraying the components with such water-miscible solvents. The components thus cleaned may also be rinsed one or more times with water either by immersion and/or by spraying. The water used for this purpose may be de-ionised water although the initial rinsing steps may be carried out with tap water. The water-miscible solvents and rinse water emergent from the respective cleaning and rinsing steps respectively can be combined or treated separately for the recovery of the solvent with a view to recylcling such solvents/water. Examples of the water-miscible solvents that may be used in the method of the present invention are suitably oxygenated solvents which are suitably high boiling and preferably have a boiling point above that of water. It is furthermore desirable that such solvents are non-flammable and are of low bio-toxicity. Examples of such solvents include inter alia alcohols, glycol ethers, glycol ether esters and natural terpenoids. More specifically these include one or more of ethoxy propanol, butyldiglycol ether, ethyldiglycol ether, ethoxypropoxy propanol, ethoxy propyl acetate and butyldiglycol acetate. Formulations containing these types of compounds are commercially available as PROZONE® and HYKLEEN® (both ex BP Chemicals Ltd) . If desired, the water-miscible solvent and/or water may be subjected to moderate heating before being used for the respective cleaning/rinsing steps. Wherever such heated solvents/water is used, the temperature should not be above the flash point of the water- miscible solvent. The electronic components thus cleaned and rinsed can then be subjected to a drying stage and will then be ready for use, storage or being packaged for despatch to a wholesale or retail outlet. The cleaned aqueous effluent may also be further treated eg by evaporation to recover the solvent. A feature of the present invention is that the aqueous effluent streams are passed through an absorptive medium which selectively removes the suspended impurities without absorbing and removing any of the valuable water-miscible solvent. Examples of such absorptive media include certain forms of carbon such as eg those derived from coal, coconut shells or wood; clays such as eg Attapulgite, Montmorillonite and Fullers Earth; polymeric fibres such as polypropylene fibres; or a combination of these. A particularly suitable absorptive medium is Attapulgite clay ODA (ex OIL-DRI) which has a particle size of 16-30 mesh (US Standard) . These absorptive media can be made in the form of permeable and replaceable filter cartridges with or without a mixed bed of deionising resin admixed therewith. Since the aqueous effluent may also contain dissolved impurities which may be cationic or anionic, it is preferable that the aqueous effluent is treated with a de-ionising resin. The de¬ ionising resin used may be a either a cationic resin or an anionic resin or a mixture of both, depending upon the nature of the dissolved impurities in the effluent. This treatment with the de¬ ionising resin may be carried out either before passing the effluent through the absortive medium or by passing said effluent through a mixed bed of such de-ionising resin and the absorptive medium. Thus, when the aqueous effluent has been freed of suspended and dissolved impurities by the method outlined above, the treated effluent comprises only the water-miscible solvent and water. This cleaned effluent can either be recycled as such or after concentration to reduce the water content thereof to one or more of the washing stages whereas the recovered water can be recycled to one or more of the rinsing stages. The present invention is illustrated more specifically with reference to the following Examples. In the Examples and comparative tests, the following materials were used: Materials Tested: Activated Carbon: All the activated carbons tested were supplied by Sutcliff Speakman Ltd, UK. The three different activated carbons tested were: a coal derived carbon grade 207A, a wood derived carbon grade 207EA and a coconut derived carbon (similar to those used in commercial water purification plants) 207C. Each of these had a particle size of 18-30 mesh (British Standard), although they had undergone different pre-treat ents. The internal pore sizes of the three materials differed with the wood and coal derived carbons having much larger pores than the coconut derived carbon. Activated Clays: Three different mineral clays were evaluated. These were: two samples each of activated Fullers Earth, activated Attapulgite and activated Montmorillonite. These are active d by the commercial supplier. The Attapulgite (LVM-GA) and Montmorillonite clays were supplied by Oil-Dri (UK) Ltd (of Bannister's Row, Wisbech, Cambridgeshire, PE13 3HZ, UK) as 30-60 mesh particles (US Standard). Both Fullers Earth were supplied by Laporte, one as a powder Fulcat® 22B and one in granular form SYK 22/44(S). Solvents: PROZONE® - a mixture of butyldiglycol ether (80%) + ethyldigylcol ner (15%) + ethoxypropoxy propanol (5%) . HYKLEEN®100 - Ethoxypropanol HYKLEEN®140 - Ethoxy propyl acetate HYKLEEN®300 - Ethoxypropoxy propanol HYKLEEN®340 - Butyldiglycol acetate EXAMPLES: In order to make the tests st- tly comparable, the granular absorptive media were evaluated in e-circulating system to simulate the way they would be used in practice. The flow rate used was set at 20 bed volumes per hour (the exact flow rate being different for each absorbent depending on the density of the material being used, cf Table 1). This gave a relatively low contact time of 3 minutes. The contaminated solvent was flowed from bottom to top through the column of the absorbent. To make the flow through the column as even as possible, a sintered glass distribution disc was used. TABLE 1 Flow Rates and Column Sizes for Absorptive Media Used The tests were carried out using a dilute solution (0.5% w/w) of contaminated Prozone®. The contaminant used for the evaluation was an unknown mixture of flux residues which had been generated during customer trials. Each absorbent was exposed to the same total volume of contaminants. The level of contamination of the Prozone® solvent was determined visually and by the use of a Lovibond® comparator to measure the Hazen colour and this measurement was then used to determine the level of flux present in the solvent. In the case of Fulcat® 22B, this was evaluated by treating a known weight of contaminated Prozone® with 1% w/w of Fulcat®22B, due to the very fine particle size of this absorbent which made re-circulating in this laboratory test virtually impossible. The treatment was carried out by the so-called "total contact method" between the solvent and the absorbent by agitation using rollers for 150 minutes. For a strict comparison, the Attapulgite and Carbon 207A samples were also tested in this manner. Results: Qualitative Evaluation: For the re-circulation method, the following observations were made. Active Carbon 207C showed no visual change in colour after 6 hours of recycling. A similar result was observed for the Fullers Earth SYK. The Active Carbon 207A produced a significant colour reduction of the contaminated Prozone as did the Attapulgite LVM-GA. Active Carbon 207EA producing an effect which was intermediate between the other two carbons. Thus the following order of merit was arrived at in terms of performance: Attapulgite LVM = Carbon 207A > Carbon 207EA > Carbon 207C = Fullers Earth SYK. The corresponding comparison for Fulcat®22B, Attapulgite LVM and Carbon 207A was carried out using the total contact method. Visual observation of the samples after 2.5, 5.0 and 10.0 hours showed the Fulcat®22B to be the most efficient absorbent for removal of flux from Prozone® with virtually no colour visible after 10 hours. There was little to choose between the other two even after 10 hours. Quantitative Evaluation: Colour was measured in Hazens using a Lovibond® Comparator and a 25 ml sample. A standard series of samples were prepared for comparison by dilution of the original with uncontaminated Prozone®. The results are shown in Table 2. These measurements confirm the visual observations on the effectiveness of the Carbons and Attapulgite. Fulcat®22B proved to be the most effective with a capacity of about 40% w/w for the flux. TABLE 2 Quantitative Measurement of Flux Residues After Treatment Absorptive Contamination Treatment Colour Type of Disc Medium (%) Time (Hrs) Hazen None 0.5 0 200-250 NSX None 0.3 0 100-150 NSX None 0.1 0 50-60 NSX None 0.05 0 15 NSA Fulcat®22B c . 0.1 10 ca. 50 NSA Fulcat®22B ca. 0.1 5 ca. 60 NSA Fulcat®22B ca. 0.1 2.5 70 NSX Carbon 207A ca. 0.3 10 100-150 NSX Carbon 207A ca. 0.4 5 150-200 NSX Carbon 207A* ca. 0.3 6 100 NSX *- Recirculation result; all others used the total contact method. Nos. in bold are approx. concentrations from the measured colours. From the above the following can be concluded: a. For optimum removal of flux residues, a coal or wood derived carbon should be used. b. A number of mineral absorbents are as effective as the Active Carbons at removing flux residues from Prozone®. c. Flux can be effectively removed from the solvent by a variety of different absorptive media provided that such media do not also absorb the solvent.

Claims: Claims: 1. A method of cleaning electronic components comprising the steps of: a. cleaning said component(s) in a water-miscible organic solvent followed by rinsing said components so cleaned at least once with water to generate an aqueous effluents cc -rising the solvent, water and suspended impurities from the cleaning and rinsing stages; and b. passing the aqueous effluent through an absorptive medium capable of removing the suspended impurities but not the aqueous solvent. 2. A method according to Claim 1 wherein the electronic components are cleaned by the solvent and rinsed with water either by sequential immersion thereof in the water-miscible solvent and water for a suitable duration and/or by spraying the components with such water- miscible solvents and water in the respective cells. 3. A method according to Claim 1 or 2 wherein the water used for rinsing is de-ionised water. 4. A method according to any one of the preceding Claims wherein the water-miscible solvents are oxygenated solvents which have a boiling point above that of water. 5. A method according to any one of the preceding Claims wherein the water-miscible solvents are non-flammable and are of low biotoxicity. 6. A method according to any one of the preceding Claims wherein the solvent is selected from the group consisting of glycol ethers, alcohols and natural terpenoids. 7. A method according to any one of the preceding Claims wherein the solvent is one or more of ethoxy propanol, butyldiglycol ether, ethyldiglycol ether, ethoxypropoxy propanol, ethoxy propyl acetate and butyldiglycol acetate. 8. A method according to any one of the preceding Claims wherein the solvent and/or reinse water used is heated to a temperature below the flash point of the solvent prior to or during use. 9. A method according to any one of the preceding Claims wherein the absorptive medium is selected from the group consisting of: i) carbon derived from coal, coconut shells or wood; ii) activated Attapulgite, Montmorillonite and Fullers Earth clays; iii) polymeric fibres; or iv) a combination of two or more of these. 10. A method according to any one of the preceding Claims wherein the absorptive medium is used in combination with a de-ionising resin whether admixed or in sequence therewith. 11. A method according to Claim 10 wherein the de-ionising resin is cationic, anionic or a mixture of the both.

Inventors: GALBRAITH DOUGLAS PETER, POOLE NEIL, GALBRAITH, DOUGLAS, PETER, POOLE, NEIL
Applicants: BP CHEM INT LTD, GALBRAITH DOUGLAS PETER, POOLE NEIL, BP CHEMICALS LIMITED, GALBRAITH, DOUGLAS, PETER, POOLE, NEIL
Application year: 1994
Main classification: B01D15
IPC codes: B01D15, C23G5, H05K3